1. Field of the Invention
The subject of the invention is a tear strip having a supporting material which is stretched in substantially one direction and optionally is provided with an adhesive layer. The tear strip is suitable in particular for cardboard acticles.
2. Description of Related Art
Self-adhesive or heat-reactivated tear strips of polypropylene film stretched in longitudinal direction have been used for some time for the tearing open of cardboard articles. They are offered on the one hand in film thicknesses of 60-65 .mu.m with a tensile strength in longitudinal direction of 230-330N/mm.sup.2, equivalent to 55-85N/4 mm of strip width. On the other hand, on a small scale, tear strips are produced in thicknesses of 120-140 .mu.m with a tensile strength in longitudinal direction of approximately 400N/mm.sup.2, equivalent to 200-225 N/4 mm of strip width.
The polypropylene tear strips in 60-65 .mu.m display a satisfactory tear behaviour with all corrugated cardboard articles, including micro-corrugated board. This finding applies irrespective of whether tearing is performed precisely in the direction of the adhesively affixed strip. "Precisely in the direction of the adhesively affixed strip" means that the tear strip is torn off in such a way that the direction of pulling never deviates from the direction which is specified by the tear strip affixed on the rear of the article and which becomes visible even from the front of the article during the tearing operation as a tear joint. Only then is the distribution of forces in the tear strip symmetrical during the tearing operation and the loading of both cut edges even. In practice, this ideal tear direction is usually not maintained. Deviations from this direction of up to approximately 30.degree. must be expected.
The polypropylene tear strips in 60-65 .mu.m thickness are suitable in the same way both for corrugated boards and for solid boards up to a basis weight of 600 g/m.sup.2 with kraftliner face layers applied on both sides on the paper machine irrespective of the tear direction. Corresponding 1000 g/m.sup.2 solid boards can only be torn open without any problems in the optimum tear direction. If there is a deviation from this direction, owing to the unsymmetrical loading of the strip, offshooting tears started from the more highly loaded edge of the strip may occur during the tearing operation. The frequency of the offshooting tears is influenced for example by inhomogeneities in the film, by micronicks in the cut edges of the film strips, by small pieces of wood or particularly compacted particles in the board and similar phenomena. In addition, the tearing speed has an effect.
In recent years, solid boards having a basis weight of between 450 and 600 g/m.sup.2 have been used to an increasing extent for the packaging of detergents, in some cases having a wax-impregnated intermediate layer, but in any case with a laminated-on face layer of high gloss cast-coated Chromolux.RTM. paper. Like the abovementioned 1000 g/m.sup.2 solid boards, these boards can only be torn open with the 60-65 .mu.m thick polypropylene tear strips with a similarly poor result.
Tear tests on corresponding cardboard articles by different test persons displayed tears in the tearing operation in 40-80% of cases. In individual cases, it was even observed that strips were torn off when torn open in the optimum tear direction. Of the known tear strips, only the 120-140 .mu.m thick tear strips with a tensile strength in longitudinal direction of approximately 400N/mm.sup.2, equivalent to 200-225N/4 mm, or thread-reinforced tear strips, which can be subjected to even higher loads, are suitable for these cases. Owing to the complex production processes and the high materials requirement, the commitment for both products is relatively expensive.
Tests have shown that a polypropylene tear strip film with a tensile strength in longitudinal direction of 320N/mm.sup.2, as are used for the 60-65 .mu.m tear strips, must have a thickness of approximately 140 .mu.m in order to achieve satisfactory results on the abovementioned detergent cardboard boxes even in the case of a tear direction deviating by 30.degree.. That is then equivalent to a load-bearing capacity of 180N/4 mm. For the expensive-to-produce film with a tensile strength in longitudinal direction of 400N/mm.sup.2, which is known from the market only in 120-140 .mu.m, this load-bearing capacity of 180 N/4 mm would still require a thickness of 112 .mu.m.
There is thus the problem of producing tear strips which, with smallest possible thickness and thus lowest possible costs, satisfactorily tear open materials having a high tearing resistance, in particular cardboard articles with which the known 60-65 .mu.m polypropylene tear strips fail.
There is in particular the problem of producing a tear strip provided with a pressure-sensitive or heat-sealable adhesive compound which has an oriented, predominantly longitudinally stretched supporting film which is predominantly of polypropylene and ensures, with smallest possible thickness, an optimum tearing result even if the tear direction deviates from the direction of the adhesively affixed strip. For mechanical processing of the tear strip, it is advantageous that a 4 mm wide tear strip is not extended by more than 10% under a load of 25N.